Ingo Molnar wrote:> * Balbir Singh <balbir@linux.vnet.ibm.com> wrote:> >> With -v7 I would run the n/n+1 test. Basically on a system with n >> cpus, I would run n+1 tasks and see how their load is distributed. I >> usually find that the last two tasks would get stuck on one CPU on the >> system and would get half the cpu time as their other peers. I think >> this issue has been around for long even before CFS. But while I was >> investigating that, I found that with -v8, all the n+1 tasks are stuck >> on the same cpu.> > i believe this problem is specific to powerpc - load is distributed fine > on i686/x86_64 and your sched_debug shows a cpu_load[0] == 0 on CPU#2 > which is 'impossible'. (I sent a few suggestions off-Cc about how to > debug this.)> > Ingo

Hi, Ingo

The suggestions helped, here is a fix tested on PowerPC only.

Patch and Description=====================

Load balancing on PowerPC is broken. Running 5 tasks on a 4 cpu systemresults in all 5 tasks running on the same CPU. Based on Ingo's feedback,I instrumented and debugged update_load_fair().

The problem is with comparing a s64 values with (s64)ULONG_MAX, whichevaluates to -1. Then we check if exec_delta64 and fair_delta64 are greaterthan (s64)ULONG_MAX (-1), if so we assign (s64)ULONG_MAX to the respectivevalues.

The fix is to compare these values against (s64)LONG_MAX and assign(s64)LONG_MAX to exec_delta64 and fair_delta64 if they are greater than(s64)LONG_MAX.

Tested on PowerPC, the regression is gone, tasks are load balanced as theywere in v7.

Output of top

5614 root 20 0 4912 784 252 R 52 0.0 3:27.49 3 bash

5620 root 20 0 4912 784 252 R 47 0.0 3:07.38 2 bash

5617 root 20 0 4912 784 252 R 47 0.0 3:08.18 0 bash

5624 root 20 0 4912 784 252 R 26 0.0 1:42.97 1 bash

5621 root 20 0 4912 784 252 R 26 0.0 1:43.14 1 bash

Tasks 5624 and 5621 getting half of their peer values is a separate issuealtogether.